Creating Detail From An Image
You could do it this way:


You can try to convert your picture in Illustrator, then import as a SVG, then convert curve to mesh. Give it thickness with an extrusion, then use the Remesh modifier to give it a good topology. Modifier settings: disable Remove Disconnected Pieces, Mode : Sharp, Octree Depth : 8, Scale : 0.3, Sharpness : 1  .
If the result is too messy, recreate your ornemental shape from scratch with your model as a Background Image. 
Give your ornement mesh a thickness with two successive extrusions on the Z axis. The first extrusion must be thin as you'll later use a Subdivision Surface modifier.
Create a plane, subdivide it to make a grid.
Give your ornement a Surface Deform modifier, choose the grid as the Target and click Bind.
Give your grid a Shrinkwrap modifier and choose the helmet as your Target. Now your ornement should stick to the helmet. Play with the Shrinkwrap Offset if necessary.
Give your ornement a Subdivision Surface modifier to smooth the shape, also, click on Shading > Smooth on the left menu of your 3D view (activated with T).


When you have finished you can choose to Bake this helmet in order to fake the ornement in the Material with a Normal map.